diff --git a/backend/problem/serializers.py b/backend/problem/serializers.py
index cf0986999..49076d718 100644
--- a/backend/problem/serializers.py
+++ b/backend/problem/serializers.py
@@ -131,7 +131,7 @@ class BaseProblemSerializer(serializers.ModelSerializer):
     def get_contest_name(self, obj):
         if obj.contest:
             return obj.contest.title
-    
+
     def get_assignment_name(self, obj):
         if obj.assignment:
             return obj.assignment.title
diff --git a/backend/problem/views/admin.py b/backend/problem/views/admin.py
index 3f02af3ef..eb917b68f 100644
--- a/backend/problem/views/admin.py
+++ b/backend/problem/views/admin.py
@@ -9,6 +9,7 @@
 from django.http import StreamingHttpResponse
 
 from contest.models import Contest, ContestStatus
+from course.models import Course
 from judge.dispatcher import SPJCompiler
 from submission.models import Submission
 from utils.api import APIView, CSRFExemptAPIView, validate_serializer, APIError
diff --git a/backend/problem/views/professor.py b/backend/problem/views/professor.py
index af1334dcf..18a65665d 100644
--- a/backend/problem/views/professor.py
+++ b/backend/problem/views/professor.py
@@ -283,7 +283,7 @@ def get(self, request):
     @admin_role_required
     def post(self, request):
         data = request.data
-        
+
         try:
             course = Course.objects.get(id=data.pop("course_id"))
             ensure_created_by(course, request.user)
diff --git a/backend/submission/views.py b/backend/submission/views.py
index c91f53d9d..091bfdc21 100644
--- a/backend/submission/views.py
+++ b/backend/submission/views.py
@@ -78,7 +78,11 @@ def post(self, request):
             return self.error(error)
 
         try:
-            problem = Problem.objects.get(id=data["problem_id"], contest_id=data.get("contest_id"), assignment_id=data.get("assignment_id"), course_id=data.get("course_id"), visible=True)
+            problem = Problem.objects.get(id=data["problem_id"],
+                                          contest_id=data.get("contest_id"),
+                                          assignment_id=data.get("assignment_id"),
+                                          course_id=data.get("course_id"),
+                                          visible=True)
         except Problem.DoesNotExist:
             return self.error("Problem not exist")
         if data["language"] not in problem.languages: